6: Intergranular Corrosion
Intergranular corrosion occurs when the grain boundaries in a metal form an anode and the interior of the grain acts as a cathode. In serious cases this can lead to the grains falling apart.
This type of corrosion is a particular problem in stainless steels, however it can also occur in other metals.
In stainless steels the problem occurs after the metal is heated to between 425°C and 870°C. During the heating, the chromium in the stainless steel reacts with carbon in the steel and forms particles of chromium carbide at the grain boundaries. The regions near the grain boundaries become depleted in chromium.
This means that the regions around the grain boundaries are no longer protected by the chromium passivation, and therefore corrode intergranularly.
Unfortunately the heating required is just that which occurs in parts of a metal during welding. Areas to each side of the weld are heated to within this temperature range. Chromium carbides form within these bands at each side of the weld. The effect is known as sensitisation.
When the area around a weld becomes sensitised, it is very susceptible to intergranular corrosion, and this is often termed weld decay.